Finger flexor rigidity in the healthy population

The involvement of the hand flexors in trigger finger is not clear. This study aimed to examine the rigidity of the flexor tendon in the first pulley territory in the hand by using ultrasound in a healthy population, as well as to create a reference scale of rigidity for the flexor tendons to compare those values in trigger fingers. We tested 35 healthy volunteers using a linear ultrasound transducer and the color Doppler method. Rigidity levels below the first pulley were examined and compared between the different fingers of the hand and the relationship between rigidity and sex and the three different age groups was evaluated. In the healthy population, the rigidity of the flexor tendons of the hand in the territory of the first pulley varied between 233.1 and 962.8 kPa, with an average of 486.42 kPa and standard deviation of 114.85. We showed that the flexors in the dominant hand were more rigid, there was a difference between the rigidity of the flexor tendons of the thumb and the other fingers of the same hand, and the ring finger of the dominant hand had stiffer flexor tendons than the fingers of the other hand in the male population. We created a value scale for the rigidity of the flexor tendons of the fingers. This base scale can be compared between different pathologies, including trigger finger. The study and all experimental protocols were approved by the local ethical committee.


Statistical analysis
Based on a pilot study, a paired samples t-test was utilized to estimate the required sample size.For a desired power of 0.80 and a Type I error rate of 0.05, the analysis indicated a need for 128 fingers (64 from each hand).This sample size would enable the detection of a mean difference of 10, considering a standard deviation of the difference of 40 in the degree of stiffness of the flexor tendon between the fingers of the right hand and the corresponding fingers of the left hand.The sample size analysis was conducted using Gpower 3.1.This study included 35 participants, with a statistical test power of at least 80%.In order to assess whether there was a difference in the average flexor tendon rigidity between the two palms, we created two variables and used a paired t-test.
Moreover, to investigate whether there was a difference in tendon rigidity between the fingers of the same hand, we used repeated measures of post hoc Bonferroni tests.
We performed paired t-tests to compare the dependent variables between the fingers of the right and left hands of the participants.The results were considered statistically significant when the p value (paired t-test) and adjusted p value were less than 0.05.All tests were two-tailed.
Regarding the three different age groups, we found that there was no significant difference in flexor tendon rigidity in the fingers of the young age group (aged 19-39 years).However, in the middle-aged group (aged 40-59 years), there was an increased variation of flexor tendon rigidity, without a specific pattern, and this tendency decreased in the older aged group (aged 60-80 years) (Fig. 5).

Discussion
In this observational study, we investigated the rigidity of hand flexor tendons through ultrasound, utilizing the SEG method, among healthy volunteers.Our objective was to establish baseline values for flexor tendon rigidity in the first pulley territory of the hand, creating a foundation for future comparisons with flexor tendon pathologies, such as trigger finger.Current knowledge suggests that in advanced pathological trigger fingers, tendons play a role in both the pathophysiological process and the narrowing of the first pulley 7,15 .
Studies from recent years have attempted to demonstrate the effectiveness of ultrasound through the sonoelastography method in revealing the degree of elasticity of tendons, particularly in the lower limbs 16,17 .
We identified that the average rigidity of the flexor tendons was higher in the right hand compared to the left hand.However, we observed no differences in tendon rigidity between the fingers of the same hand.Additionally, US longitudinal posiƟon of the pulley A1 and the flexor tendons.the flexor tendons of the thumbs exhibited distinct characteristics compared to those of the other fingers within the same hand; however, no significant difference was found between the flexors of the thumbs of the two hands.This finding may be explained by the fact that the thumb has one flexor tendon, whereas the other fingers have two.
Of note, in the male population, the flexor tendons in the ring finger of the dominant hand (which was the right hand for the majority-94%-of the subjects) were more rigid than the flexor tendons of the left-hand fingers, excluding the thumbs.This phenomenon could be attributed to the function of the dominant hand's ring finger in daily activities, especially considering that men typically engage in more substantial gripping activities  www.nature.com/scientificreports/than women.However, it's essential to consider previous research of Radhakrishnan et al. 15 indicating greater isometric strength in the index finger compared to the fourth finger 16 , warranting further investigation.Moreover, when stratifying subjects into different age groups, the middle-aged group displayed a greater tendency for variation in flexor tendon rigidity, without a discernible pattern.This variability decreased in the younger and older age groups.This observation may be explained by the higher level of function and activity in middle-aged individuals, with long-term effects on tendon stiffness due to cumulative fingers activities.If this theory holds, we anticipate that the older age group would exhibit greater tendon rigidity, necessitating further investigation.
Several limitations were inherent in this study.The statistical test power was 80%, owing to the overall small sample size and the limited number of healthy volunteers in the older age group.Additionally, most participants had a dominant right hand, influencing the generalizability of the findings.Another limitation pertains to a single examiner conducting the ultrasound examination, without a comparison of results among different examiners.
Despite these limitations, this study holds clinical significance and has the potential to serve as a cornerstone for future investigations into flexor tendon pathologies.Furthermore, our results may guide surgeons in selecting  Looking forward, these findings may equip hand surgeons with an additional non-invasive tool to aid in decision-making regarding the necessity of specific surgical interventions for patients with "trigger finger".This could include determining whether a patient requires only a release of the first pulley or a combined surgery, such as resecting half of the superficial flexor tendon 17 and releasing the first pulley, especially when presurgical ultrasound tests indicate a high level of rigidity.
LeŌ picture: B mode picture: Pulley A1.Proximal phalanx.Metacarpal head.Flexor tendons.Volar plate.Right picture: Elastography at the same height as the B mode picture.The color represents Ɵssue rigidity in the examined area, the scale is between red/yellow for soŌer components and blue for more rigid components.

Figure 1 .
Figure 1.The flexor tendon in a longitudinal section below the pulley A1 in an ultrasound examination and elastography (measuring rigidity).

Figure 2 .
Figure 2. Comparison of the average rigidity of the flexor tendons of between the two hands.

Table 2 .Figure 3 .
Figure 3.Comparison of the average rigidity of the flexor tendons between the fingers of the hands.

Figure 4 .
Figure 4.The average rigidity of the flexor tendons of the fingers of the hands by gender.

Figure 5 .
Figure 5.The average rigidity of the flexor tendons of the fingers of the hands by age groups.

Table 1 .
Characterization of the participants in the study and the mean flexor tendon stiffness and standard deviation.

Table 3 .
Comparison between flexor tendon of the right ring finger and the flexor tendons in each finger of the left hand.
Pairs (R